1. Field of the Invention
The present invention relates to loudspeaker diaphragms and, more specifically, to a loudspeaker diaphragm for use in a loudspeaker which is expected to be reduced in thickness.
2. Description of the Background Art
In recent years, electronic devices, such as cellular phones and PDAs (personal digital assistants), have been slimmed down and equipped with a larger screen, while still being expected to produce high-quality sound. Accordingly, loudspeakers incorporated in such electronic devices are similarly expected to be slimmed down and to still produce high-quality sound. To fulfill such expectations, rectangular- or oval-shaped slim loudspeakers have been suggested.
FIGS. 12A and 12B are illustrations showing the structure of a conventional slim loudspeaker. FIG. 12A is a top plan view of the conventional slim loudspeaker, and FIG. 12B is a front elevational view thereof. In FIGS. 12A and 12B, the slim loudspeaker includes a magnet 901, a plate 902, a yoke 903, a housing 904, a cylinder-shaped coil 905, and a diaphragm 906 shaped like an oval. Located on a center portion of the diaphragm 906 (the portion surrounded by the coil 905 affixed to the diaphragm 906) is a dome-shaped portion 911 shaped like a semi-circle in cross section. Furthermore, located at an outer rim of the diaphragm 906 (a portion outside of a dotted line drawn on the diaphragm 906 of FIG. 12A) is an edge portion 912 shaped like a semi-oval in cross section. The edge portion 912 of the diaphragm 906 is supported by the housing 904. Here, the diaphragm 906 is supported so that the coil 905 is inserted in a magnetic gap between the plate 902 and the yoke 903.
In FIGS. 12A and 12B, the coil 905 is shaped like a circle when viewed from above. Therefore, it is difficult for the driving force of the coil to propagate in the direction of the length of the diaphragm 906 (in the horizontal direction in FIG. 12A). To prevent this difficulty, the coil can be shaped like an oval, as is the diaphragm. With this shape, the rigidity of the diaphragm in the direction of the length thereof can be maintained. On the other hand, in order to sufficiently ensure the rigidity of the diaphragm in a direction perpendicular to the direction of the length thereof, the center portion of the diaphragm is strengthened by shaping the center portion like a dome, as illustrated in FIG. 12B, or by using a voice coil bobbin in the conventional slim loudspeakers.
However, such a dome-like portion or a voice coil bobbin required to strengthen the center portion of the diaphragm disadvantageously increases the height at the center portion. Therefore, there is a limit in the conventional structure to slim down the diaphragm. Moreover, particularly in the case of strengthening the diaphragm by a voice coil bobbin, a vibrating system of the loudspeaker is increased in mass, thereby decreasing pressure sensitivity.
In the conventional structure of the diaphragm shaped like an oval or a rectangle, the elasticity in the vicinity of the center portion of the diaphragm is different from the elasticity in the vicinity of both ends of the width of the diaphragm. Specifically, the elasticity in the vicinity of the center portion is small, while the elasticity in the vicinity of both ends is large. As a result, the diaphragm performs a drum-like motion, in which a larger amplitude is observed at portions closer to the center portion (refer to FIG. 8). That is, it is not possible for the entire diaphragm to perform a piston motion. This causes a problem of degradation in sound pressure frequency characteristics.